1. Field of the Invention
The present invention generally relates to a method for selective formation of trenches. In particular, the present invention relates to a method to change the etching selectivity of a substrate by dopant prior to selective formation of trenches around a semiconductor element without the protection of a mask.
2. Description of the Prior Art
Generally speaking, in a semiconductor manufacturing process, an additional mask is needed to protect the locations which should remain intact when trenches are needed to be selectively formed in some places in a substrate. FIGS. 1-3 illustrate the conventional way to form trenches at some places of a substrate. As shown in FIG. 1, first, a substrate 101 is provided. There are a P-type semiconductor element 110 and an N-type semiconductor element 120 previously formed in different regions of the substrate 101. There is also a shallow trench isolation 130 disposed between the P-type semiconductor element 110 and the N-type semiconductor element 120 to segregate the P-type semiconductor element 110 and the N-type semiconductor element 120.
Now, as shown in FIG. 2, when trenches are needed to be selectively formed in the substrate 101 around the P-type semiconductor element 110, as described earlier, a mask 140, such as a photoresist, is used to cover the corresponding region around the P-type semiconductor element 110 to keep the P-type semiconductor element 110 from the damages caused by the coming etching step. Next, as shown in FIG. 3, the pre-determined etching step is performed, such as a dry etching procedure, to form the expected trenches 150 in the substrate around the N-type semiconductor element 120.
However, in order to form the mask 140 to cover the corresponding region around the P-type semiconductor element 110 to keep the P-type semiconductor element 110 from the damages of the following etching step, another new reticle must be designed. However, it is well known that it costs a lot of money to design and to make a new reticle. Accordingly, the additional reticle is a heavy cost burden to the manufacturer of the semiconductor. Besides, even though a dry etching procedure has a faster etching rate, it inevitably makes the etching procedure less easily and evenly controlled. In the light of this, people of ordinary skills in the art would understand that the current way to selectively form trenches at some places of a substrate is still possible to be improved.